Global navigation satellite systems such as GPS and GLONASS, now deployed by the United States and Russia, represent a revolutionary change in navigation and positioning technology. One of the areas affected profoundly by the availability of satellite-based navigation (SatNav) is civil aviation, where these systems show an enormous promise for enhancing economy as well as safety. Planning is underway in the U.S. to switch to GPS-based navigation and surveillance in all phases of flight, and government approval to do so has been received.
Satellite navigation provides the user with estimates of the three coordinates of the user's position: x, y, and z. System receivers measure and analyze signals from satellites, and estimate the corresponding three coordinates of the receiver position, as well as the instantaneous receiver clock bias. The quality of these estimates depends upon the number and the geometry of satellites in view, and whether the satellites are operating properly. Conventional receivers operating with satellite navigation systems measure the transit time of a signal from a satellite to determine the distance or range from the satellite to the user. This range is determined mathematically by the speed of light multiplied by the signal transit time.
If receiver clocks were perfectly synchronized with the satellite clocks, only three range measurements would be needed to allow a user to compute a three-dimensional position. This process is known as multilateration. However, given the expense of providing a receiver clock whose time is exactly synchronized, conventional systems have found a way to account for the amount by which the receiver clock time differs from the satellite clock time when computing a user's position. This amount, hereinafter referred to as the clock bias, has been determined by computing a measurement from a fourth satellite using a processor in the receiver that correlates the ranges measured from each satellite. This process requires four or more satellites from which four or more measurements can be obtained to estimate four unknowns x, y, z, b. The processor can be programmed either to subtract from or add time to all of the measurements, continuing to do so until it satisfies a three-dimensional mathematical model of the satellite positions, the user's position, and the measured transit times. The amount b, by which the processor has added or subtracted time is the instantaneous bias between the receiver clock and the satellite clock. The range measurements with a common offset due to this bias are referred to as pseudoranges.
Obtaining measurements from four satellites in view does not necessarily assure a good position estimate. The quality of a position estimate largely depends upon two factors: satellite geometry, particularly, the number of satellites in view and their spatial distribution relative to the user, and the quality of the pseudorange measurements obtained from satellite signals.
Satellite geometry is characterized by a parameter called Dilution of Precision (DOP). This parameter, DOP, can be thought of geometrically as roughly inversely proportional to the volume of a polyhedron with the receiver being at the apex and the satellite positions defining the base. Generally, the larger the number of satellites in view and the greater the distances therebetween, the better the geometry of the satellite constellation. The quality of pseudorange measurements is affected by errors in the predicted ephemeris of the satellites, instabilities in the satellite and receiver clocks, ionospheric and tropospheric propagation delays, multipath, receiver noise and RF interference. The collective effect of these errors is referred to as the User Range Error (URE). The root mean square (rms) value of the User Range Error is expressed as .sigma.URE. The error associated with the position estimate can be expressed as a rms value calculated in terms of these factors; .sigma.URE and DOP, and represented mathematically by RMS position error=(DOP)( .sigma.URE).
In order to avoid the hazards associated with erroneous position estimates, civil aviation regulations require that the integrity of the navigation system be monitored and that the user be warned when a position estimate obtained from it should not be used. In satellite navigation, such integrity monitoring, when performed within a receiver, is referred to as receiver autonomous integrity monitoring (RAIM). Conventional methods for carrying out RAIM require that at least five satellites be in view in order to detect the presence of faulty signals, and that at least six satellites be in view in order to identify a faulty signal.
Additionally, the constellations of GPS and GLONASS are such that the number of satellites in a user's view would depend upon a user's location and would change with time due to the movement of satellites. At certain times, some of the users may have fewer than five satellites in view. As a result, these users will not be able to perform RAIM, and thus will not be able to rely on position estimates obtained from these satellite systems.